Telephone-exchange system



Filed Dec. 15. 1923 2 Sheets-Sheet 1 h CEZ W L d E n 2 6 a 3 V. 4 mf orwwh 5 u n J 1927. Jan 4 J. F. DAHL TELEPHONE EXCHANGE SYSTEM Filed Dec. 15 1923 2 Sheets-Sheet 2 IiEUQEL MNN v NQN:

Patented Jan. 4, 1927.

I UNITED STATES PATENT OFFICE.

JACK F. DAHL,

COMPANY, INCORPORATED,

change system, and mute for extending toll on EAST ORANGE,

NEVJ JERSEY, ASSTZGNGB T O WESTERN ELECTRIC 0F NEW YORK, N. L, A GGB?GRATION OF NEW YORK.

TELEPIIONE-EXCHANGE SYSTEM.

to a telephone exconnections from toll points to a manual terminating ofiice functioning on a call indicator basis.

In exchange areas in which certain offices are equipped with mechanical switching mechanism for establishing calls either local thereto or. calls to other oflices,. and in which other Ol'llCGS are equipped manually, the manual to extend calls offices are provided with call indicator apparatus for recording designations of lines in mechanical offices with which subscribers desire connections. The

call indicator apparatus of such manual offices is also adapted to lines to which connec record designations of tions are desired by subscribers in other manual ofiices of the exchange area.

operators in such other positions are provided trunks incoming Such calls are initiated by the manual o-fiices whose with key indicator are designed to funcbasis, it is also desirthereto from toll connections may be handled and terminated in substantially the same manner as local points may be located areas, and therefore cator senders or may Such toll exchange calls. in manual equipped with key indibe of a character such that the toll operators are provided with outgoing dialling trunks.

Regardless of the character of the toll point in which the toll connection originates. able the extension of for termination in manual indicator type.

it is desirable to ensuch toll connections oflices of the call It is therefore the object of the invention to enable toll calls originating in toll points of different characters to be quickly and efficiently extended over a common trunk to a manual terminating oiii A feature of the invention is the provision of a trunk circuit terminating at its incoming end at a call indicator operators position, which may be taken for use at its out going end by either a key indicator apparatus or by selector which is set upon the trunk circuit by a dialling over toll operator having a toll district the terminals of distant toll operator a toll line.

A further feature resides in the provision of means for rendering the trunk circuit busy to prevent connec tion thereto by either Serial No. 680,812.

a toll district selector or by a key indicator link whenever the trunk circuit has been taken'for use either by another toll district selector or another key indicator link.

A further feature of the invention resides in the arrangement of a permanent lock-up circuit arranged so that the trunk can only be released and rendered accessible for other toll connections when the toll operator or toll district selector disconnects therefrom and the called subscriber in the terminating oiiice has either restored his receiver to its switchhook or the terminating operator has taken d wthe connection. I

A still further feature resides in the provision of a high impedance bridge across the runk circuit for preventing a false disconnect signal and release of the call indicator trunk at the time the toll operator opens her cord bridge in operating her ringing key for controlling ringing.

A still further feature of the invention relates to the provision of means for enabling the toll operator to initiate the application of ringing current at the terminating office, the ringing current being applied to the tip or ring side of the called line in accordance with a single or two ring code, and to enable the toll operator to rering if desired.

These and other features of the invention accompanying drawings and the appended claims.

Referring to the drawings, Fig. 1 shows at the right a trunk circuit outgoing to a manual call indicator office and accessible from a toll point shown in the upper left portion of the figure or a toll point of different character, as shown in the lower left portion of the figure. In the lower right corner of the figure, a key indicator link is diagrammatically designated. This link is under the control of the toll operator whose position is shown in the upper left corner of the figure.

Fig. 2 shows the trunk of Fig. 1 incoming to a manual call indicator office and terminating'in a calling plug, a party line terminating at the manual. office and a diagraininatic representation, as shown in the small dot-ted rectangle of a call indicator.

Two types of toll points are disclosed in Fig. 1, one shown in the upper left portion ofthe figure coinprlsing a toll operators toll cord has been shown as having access directly to a calling line for answering calls initiated thereby, it would in practice be associated with the usual A and recording operators positions and calls would be handled in accordance with standard toll practice. In order to simplify the illustration, however, the usual A and recording operators positions have been omitted and it has been assumed that the toll operator at position O directly answers calls initiated by subscribers. The toll point at which the operators position 0 is located is provided with key indicator mechanism, diagrammatically disclosed in the dotted rectangle in the lower right corner of Fig. 1, which may be of the type disclosed in the application of A. C. Powell, Serial No. 678,141, filed December 3, 1923. This key indicator mechanism comprises a plurality of officesenders, a key board for each toll operators position and a plurality of links for associating key boards, senders and outgoing trunk circuits with each other.

The toll line 174 and the toll district selector 17 5, shown in the lower left portion of Fig. 1, are employed for establishing toll connections from a distant toll point and may be of the general type disclosed in the application of V. C. Jordan, Serial No. 680,807, filed Dec. 15, 1923. Since the specific construction of these switches and the functioning thereof are immaterial to an understanding of the invention, a detailed disclosure thereof is unnecessary herein.

The call indicator trunk shown in Fig. 2 and the call indicator associated therewith are of substantially the same type as disclosed in Patent No. 1,464,084, issued to A. E. Lundell and H. Clark, August 7, 1923, the trunk circuit being slightly modified in accordance with this invention. The call indicator has been disclosed diagrammatically in the small rectangle in the lower portion of the figure, and reference to the aforementioned Patent No. 1,464,084 may be made for a full disclosure thereof.

It is thought that the invention will be best understood from a detailed description of the manner in which toll connections may be established. It will first be assumed that the calling subscriber A desires to establish a toll connection with the line of subscriber B, this connection being established through a toll operators position 0. The calling subscriber A upon initiating a call becomes connected with the operators position O in the well known manner. The operator at position 0 in answering the call inserts the plug 104 of her cord circuit in the jack 103, connecting her cord circuit with the calling line. Having obtained instructions that a toll connection is desired with line B, the operator thereupon depresses an office key of the key indicator mechanism, which initiates the hunting movement of a trunk finding switch 150 associated therewith to select an idle trunk circuit extending to the officc in which the line of subscriber B terminates.

The trunk selector 150 hunts over the terminals of trunks in the group extending to the desired ofiice until the brush 125 encounters a test terminal upon which there is no ground potential. At the time the trunk selector 150 is searching for an idle trunk in the desired group, an idle sender is being selected and the key board of the calling toll operators position is being associated with the selected idle sender, all in the manner fully disclosed in the application of W. C. Jordan hereinbefore referred to. As soon as the idle trunk is found, the toll operator is given an indication of the number of the selected trunk. She thereupon inserts the calling plug 106 of her cord circuit in the jack 107 of the indicated trunk circuit.

l/Vith the plug 106 in the jack 107, a circuit is established from battery through the relay 100, sleeve contacts of plug 106 and jack 107 to ground through the winding of relay 108. Relay 108 energizes in this circuitand at inner left armature connects ground to the conductor 126 for marking the trunk busy to other key indicator links, which may attempt to make connection therewith. Ground potential is also connected over this armature and through the back contact of the innermost left armature of relay 110 to the multiples of test terminal 111, appearing in the banks of toll district selectors 17 5 for marking the selected trunk busy to all toll se ectors having access thereto. Upon the insertion of the plug 106 in the jack 107, a circuit is also established for relay 109, which may be traced from battery through the winding of relay 109, the innermost lower armature of relay 119, the outer left armature and back contact of relay 110, tip contacts of jack 107 and plug 106, winding of relay 120, armature and contact of relay 100, rin contacts of plug 106 and jack 107 to ground at the upper armature back contact of relay 119. The supervisory relay energizes in this circuit and causes the illumination of the lamp 121 in an obvious manner. Relay 109 upon energizing establishes a circuit for relay 112 which may be traced from battery at the key indicator link, over brush 113 of the trunk selector 150, conductor 114, the right armature and back contact of relay 110, the outer left armature and contact of relay 108, the armature and contact of relay 109, the right normal contacts and winding of relay 112 to ground at the inner left armature of relay 108. Relay 1 2 upon energizing in this circuit locks directly to battery over its right armature and alternate contact and to ground at its inner left armature and contact, opening the circuit by which it was initially energized, thereby rendering its energization independent of the continued energiz'a'tion of relay 109. Busy ground potential is also immediately applied over the inner left armature contact of relay 112, and the inner left armature and back contact of relay to the testmultiples of terminal 111 to prevent seizure of the trunk by toll dialing districts such as 17 5, and to conductor 128 to prevent seizure by another key indicator link. At this time, the mechanism associated with the key indicator link has advanced to such a point as to connect the stepping relay 114: of the sender over the brushes 115 and 116 of the trunk selector 150, with conductors 117 and 118 of the trunk circuit, whereupon the line relay 200 at the distant end of the trunk circuit is energized. The circuit for relay 200 may be traced from battery through the left winding of relay 200, the left armature and back contact of relay 201, the upper left winding of repeating coil 202, the upper armature and back contact of relay 203, conductor 117, brush 115, winding of stepping relay 114, brush 116, conductor 118, the intermediate armature and back contact of relay 203, lower left winding of repeat ing coil 202, inner right armature and back contact of relay 201 to ground through the right winding of relay 200. Relay 200 upon energizing establishes a circuit for relay 20% extending through the left armature and back contact of relay 211 to ground at the armature contact of relay 200. Relay 204: at its inner right armature and front contact establishes a circuit for the assignment lamp 205 extending to battery at the inner right armature and back contact of relay 209 if the plug 214 is down, or to battery through the outer right armature and front contact of relay act if the plug 21 1 is up at the time. Relay 20s at its left armature and contact prepares a circuit for the assignment relay 207.

The manual terminating operator upon noting the illumination of lamp 205 depresses the display key 206, establishing a circuit for relay 207, which may be traced in part from the relay call indicator through the upper contact of key 206, the left armature contact of relay 20%, the right winding of relay 207, conductor 208, outer right armature and contact of relay 209,

- conductor 210 to the relay call indicator.

Relay 207 energizes in this circuit and looks from ground over its inner left armature and contact through both of its windings, thence over conductor 208 to battery at the relay call indicator. Upon the energization of relay 207, a circuit is established from battery through the winding of relay 203, the outer left armature and contact of relay 207 to ground at the relay call indicator for associating the conductors 117 and 118 of the tact opens the circuit of trunk with the relay call indicator. Relay 203 at its lowermost armature and contact also establishes an obvious circuit for relay 211 which at its left armature and back conrelay 204C and at thetfront contact of this armature locks to ground at the contact of relay 200. lVith relay 204 deenergized and relay 207 energized, the circuit of lamp 205 is disconnected from direct ground at the inner right armature and front contact of relay 20a and connected through this armature and its back contact, and the right armature and front contact of relay 207 to interrupted ground for the purpose of fl shing the lamp 205 during the setting of the relay call indicator.

Relay 203-upon energizing also opens the previously traced circuit of relay 200 but due to the energization of relay 211, a local circuit is established for relay 200 before it has time to retract its armature. This circuit extends through the windings of relay 200 and the inner front contacts of the armatures of relay 201, the relay 201 becoming energized upon the energi-zation of relay 211 over a circuit extending from battery through its right winding, the innermost right armature and contact of relay 211 to ground at the left armature and back contact of relay 209. With relays 201 and 211 both energized a circuit is also established from battery through the right winding of relay 229, outer right contact of relay 201, inner right lay 211 to ground at the left back contact of relay 209. Relay 229 operates but is without function at this time.

After all impulses have been sent for set tin the relay call indicator in accordance with the designation of the desired line, battery potential is removed at the key in'dicator from the brush 113 and the shunt existing up to this time around the upper winding of relay 119 is removed. Ground potential is then connected to brush 113, and relay 119 now energizes over a circuit extending from battery through its upper winding, the inner left armature of relay 112 to ground on the brush 113. Relay 119 upon energizing, locks over its lower winding and lowermost armature contact to ground at the outer left armature contact of relay 112, and at its upper armature and back contact and at its innermost lower armature and back contact opens the circuit of relays 109 and 120. At the front contacts of these armatures, the trunk conductors 117 and 118 are connected through to the ring and tip contacts respectively of jack 107. Relay 112 being looked over a circuit extending from battery through the right armature and front contact of relay 112 to ground at the innermost left armature and front contact of this relay and relay 119 being looked as new contact of redescribed over the outer left armature of relay 112, these relays remain locked until relay 112 is shunted through the operation of relay 123 as will hereinafter appear.

At the terminating operators position, following the complete transmission of impulses for setting the relay call indicator, relay 203 becomes deenergized in the well known manner disconnecting the call indicator from conductors 117 and 118 of the trunk circuit and connecting these conductors to the left windings of repeating coil 202. Since at this time relay 201 is energizer and battery and ground connections are disconnected from the conductors of trunk cir cuit 117 and 118, the deenergization of relay 203 is not effective at this time to cause a false operation of the supervisory relay 120 of the cord circuit O.

The terminating operator noting the number displaced on her call indicator, tests the jack of the indicated line with the tip of plug 214 to determine if the line is idle or busy. Assuming tha the line tests idle, she then inserts the plug 214- in the 215, whereupon a circuit is extended from ground through the winding of cut-off relay 226, sleeve contacts of ack 215 and plug 214:, winding of relay 216 to battery through the winding of relay 217. Each party line i. provided with four jacks, a jack corresponding to each of the stations on the line. The sleeves of two of these jacks are connected to ground through the cut-otf relay and through a low resistance and the other two jacks through a high resistance. 1n addition, the tip and ring multiples of two of the iacks are reversed so that four different solective conditions may be established in the connecting cord circuit for determining the characteristics of the ringing current to he applied to the line and for determining to which side of the line the ringing current shall be applied. Thus, the operator in connecting with the line inserts the plug in the jack corresponding to the station indicated by the stations lamps of the call indicator.

1f the sleeve resistance is low, both the relays 216 and 217 will operate when the plug 214 is inserted in the jack. If, however, the sleeve resistance is high, only relay 216 operates. Thus, the energized or deenergized condition of relay 217 determines whethe' ringing current of one character 0%; of another character is to be applied to the line. In the particular case under consid eration, the character of the ringing current is altered by applying i tarot ferent interrupters and rupter 22a applying the ringing current accordance with a two-ring code and the terrupter 225 applying it in accordance w a one-ring code.

Relay 216 upon energizing closes an obvious circuit for relay 209 which, at the back contact of its inner rightarmature, opens the circuit of lamp 205, thereby extinguishing it. At the front contact of this armature, a circuit is prepared for the disconnect lamp 218. At the contact of .its outer right armature, relay 209 opens the locking circuit of relay 207 which thereupon deenergizes. At the hack contact of its left armature, relay 209 opens the circuit previously traced through the right winding of relay 201 which thereupon deenergizes opening the circuit of relay 229 and connecting the windings of relay 200 to the conductors 117 and 118 of the trunk circuit causing the energization of supervisory relay 120 of the toll operators cord circuit and the illumination of lamp 121 as a ringing signal to the toll op erator.

In response to the ringing signal, the toll operator depresses ringing key 122, applymg ringing current to the tip and ring strands of her cord circuit, whereupon ringreiay 212 operates over a circuit extend ing from the source of ringing current 128, the lower contact of the ringing key 122, ring contacts of plug 106 and jack 107, upper re and contact of relay 119, conductor armature and back contact of releft winding of repeating coil 20:), upper windi; g of ringing relay 212, condenser lower left winding of repeating coil intermediate armature and back conact of relay 203, conductor 118, innermost ower armature and front contact of relay 19, outer left armature and contact of rea 110, tip contacts of jack 107 and plug 08 to ground at the upper contact of ringg key 122. At this time, the tip and ring ontacts of jack 107 are bridged through intermediate lover armature of relay high impedance coil 124': and the right armature and front contact of relay 108, for the purpose of preventi the release of relay 200 and the consequent operation of disconnect lamp 218, and release of the terminating opcrators circuit due to the opening of the toll operators cord circuit at the time depresses ringing key 122. Upon the energi'; tion of relay 212, an obvious circuit tblished through the left winding of .01 which establishes a local holding for relay 200 for preventing the ree of relay 200 and the production of a disconnect signal. The previously traced cir uit for relay is not established at this time since relay 209 is now energized. In irallel with the circuit through the left winding of relay 201, a circuit is established through the upper winding of ringing relay 220. Relay 220 upon energizing locks irou h its lower winding, its innermost ture, the right armature and back e met of relay 219, the outer right armature and contact of relay 211, the outer right back contact of relay 229, to ground at the ill left armature and front contact of relay 209. As soon as key 122 is released relay 212 deenergizes and releases relay 201 which reconnects battery and ground through relay 200, to the trunk circuit. Relay 120 again operates to illuminate lamp 121.

It will first be assumed that the plug 214 has been inserted in the jack corresponding to a station which is to receive ringing current in accordance with a two-ring code and that therefore relay 217 is at the time deenergized. As, soon as ringing relay 220 en ergizes, a circuit is established from ground through its innermost upper armature and contact, the left armature and contact of relay 219 winding of pick-up relay 228, the normally closed right armature contacts thereof, the right armature and back contact of relay 217 tobattery, through the pick-up interrupter 222. Relay 228 upon energizing closes a locking circuit over its right armature and front contact independent of the interrupter 222. A circuit is now established for applying ringing current from the source 223 to the called line through the interrupter 224 over the left armature and front contact of relay 228, the left armature and back contactof relay 217, the ringing cut-ofi' relay 221, the inner right armature and back contact of relay 229, the lowermost armature and front contact of relay 220, ring contacts of plug 214 and jack 215 through the call bell of the substation B to ground. I This current is applied 'to the called line in accordance with a tworing code until the called subscriber responds, whereupon relay 221 energizes opening a shunt around the winding of relay'219 which extends through the back contact of relay 221 and the outer right back contact of relay 229. Relay 219 thereupon energizes over a circuit from battery through its windings, the outer right back contacts of relay 229, to ground at the left armature and front contact of relay 209 and opens the locking circuit of relay 220 and the locking circuit of relay 228, whereupon these relays deenergize. As soon as relay 220 deenergizes, the ringing current leads are disconnected from the strands of the cord circuit and the tip and ring contacts of plug 214 are connected through to the right windings of repeating coil 202.

A circuit is now established for supervisory relay 227 extending from battery through the winding of this relay, lower right winding of repeating coil 202, lowermost back contact of relay 220, ring contacts of plug 214 and jack 215, thence over the called subscribers line loop and returning through the tip contacts or jack 215 and plug 214, the upper armature and contact of relay 216, the upper armature and back contact of relay 220 to ground through the upper right winding of repeating coil 202. Relay 227 upon energizing estabilshes a circuit through the right winding of relay 20.1 to ground through the intermediate right armature and front contact of relay 211, causing the energizati on of relay 201, which thereupon disconnects the windings of relay 200 from the conductors of the trunk circuit for the purpose of releasing relay 120 and extinguishing lamp 121 at the toll operators position, indicating that the called subscriber has responded.

Had the operator plugged into the jack corresponding to a substation designed to be called by a one-ring code, then the relay 217 would have energized in series with relay 216. In this case, the pick-up relay 228' does not operate and ringing current is applied from source 223, through the one-ring code interrupter 225, the left armature and front contact of relay 217 and thence as previously traced.

- Ringing current is applied over the two jacks of the other two substations of the called line in the same manner as previously described, except that the tip and ring conductors being reversed the ringing current is applied to the other side of the line.

On completion of the conversation, the toll operator receives a disconnect signal from the calling subscriber and removes the plugs 104 and 106 from the jacks 103 and 107 respectively. As soon as plug 106 is removed from jack 107 relay 108 deenergizes closing a circuit for bridging relay 123 across conductors 117 and 118 of the trunk circuit. If at this time the called subscriber has restored his receiver to the switchhook and relays 227 and 201 are deenergized, the circuit of relay 123 is completed from bat tery through the left winding of relay 200, the left armature and back contact of relay 201, the upper left winding of repeating coil 202, the upper armature and back contact of relay 203, conductor 117, the upper armature and front contact of relay 119, the intermediate lower armature and contact of re lay 119, winding of relay 123, right armature and back contact of relay 108, the outer left armature and back contact of relay 110, the innermost lower armature and front contact of relay 119, conductor 118, the intermediate armature and back contact of relay 203. the lower left winding of repeating coil 202, the right armature and back contact of relay 201 to ground through the right winding of relay 200. Relay 123 upon operating connects ground to both terminals of relay 112 shunting this relay and causing its deenergization. Relay 112 upon deenergizing opens the locking circuit of relay 119 which in turn releases. With relays 108 and 112 both deenergized, the ground potential is removed from the multiples of terminal 111 and conductor 126, whereupon the trunk circuit is made available for other toll connections. As soon as relay 119 deenergizes, the circuit previously traced through relays 123 and 200 is opened and these relays release, relay 200 in turn opening the locking circuit of relay 211. With relay 211 deenergized, a circuit is completed for the disconnect lamp 218 extending from ground through the intermediate right armature and back contact of relay 211, lamp 218, to battery through the inner right armature and front contact of relay 209. The operator noting the illumination of the disconnect lamp 218 removes the plug 214 from the jack 215, whereupon relays 226, 216, 217, 209 and 219 release and lamp 218 becomes extinguished. All of the apparatus employed in the establishment of the connection is now restored to normal condition.

If it is desired to rering the called subscriber after he has restorec. his receiver to its switchhook relays 200, 211, 210, 209, 228 and 219 being still energized, and before the toll operator has removed the plug from the jack 107, the toll operator may initiate reringing by again depressing the ringing key 122. In response to the operation of key 122, ringing relay 212 becomes energized, in turn causing the operation of relays 201, 229 and 220. Relay 229 upon energizing opens the circuit of relay 219 and this relay deenergizes. Since, relay 219 is deenergized and relay 229 deenergizes as soon as the ringing key is released and releases relay 212, relay 220 looks as before and rin ing current is applied from the source 223 in the same manner as before, continuing until the subscriber again responds.

Had the connection been established to a private branch exchange trunl-t 280, the ringing operations for initially ringing out on the trunk would have proceeded in the same manner as hereinbefore described, except that the ringing trip relay 221 would operate to trip the ringing upon the response of the operator and relay 227 would operate from a bridge in the operators cord circuit. During conversation and long as the branch exchange operators cord circuit 231 is up on the aclr 232 of the branch exchange trunk 230, relays 200, 21.1. 216. 209, 219 and 227 remain operated. If the toll operator desires to rering the branch exchange operator she depresses her ringing key 122, causing the operation of relay 212 which in turn causes the operation of relays 201 and 220 as before. Since, however, relay 22': is at the time energized, a circuit is also established for relay 229 from battery, right winding of this relay, outer right contact of relay 201, contact of relay 227 to ground at the intermediate right contact of relay 211. Relay 229 upon energizing opens the circuit of relay 219 whereupon relay 219 deenergizes. With relay 229 energized the locking circuit of relay 220 extending over the right quipped with means for dialing over the toll line, the toll line is connected with the trunlr circuit 117 and 118, through the operation of the toll line-switch 174 and the toll district selector 175, the districtselector 1. 5 being set upon the terminals of the trunk circuit 117, 118 under the control of an oiiice sender associated with the district selector. The registers of this sender are set to record the designation of the desired line through the operation of the d stant toll operators dial. If the trunk circuit 117, 118 is idle at the time the toll selector 175 makes connection therewith, a circuit is extended over the tip and ring brushes of the selector 175 for relay 135, this circuit being traced in part from battery through the winding of relay 135, the innermost upper armature and back contact of relay 130, thence through the toll selector returning to ground at the lower armature and bacl; contact of relay 136. With relay energized, a circuit is now established from ground over the sleeve conductor and test brush of selector 1T5, terminal 111, the armature and contact of relay 135 to battery through the winding of relay 110. Relay 110 at its inner left ari'na-ture and frontcontact loclrs directly to the sleeve conductor irrespective of the contacts of relay 135. At the alternate contact of its inner left armature, relay 110 also connects busy ground potential to conductor 126 for marking the trunk circuit 117, 118 as busy to the key indicator links having access thereto. At the contact of its intermediate left armature, relay 110 also establishes an obvious circuit for the transfer relay 186 which e):- tends the toll line through the brushes of lineswitch 17 i and the brushes of selector 175 to conductors 117 and 118 of the trunlr c'rcuit. v

With relays 110 and 136 energized, a circuit is established from battery through the upper armature and contact of relay 186, the right normal armature contacts of relay 112 to ground at the inner left armature and front contact of relay 110. Relay locks to battery and ground at its right and inner left armature contacts. A circuit is also closed for relay 119 from battery through its lower winding to ground at the upper intermediate contact of relay 136,

ill

relay 119 lockingover-contacts of relay 112 in the manner previously described. Relays 112 and 119 are maintained locked until relay 123 becomes energized following the release of relay 110 when the selector 175 disconnects from the trunk circuit, for the purpose of preventing the seizure of the trunk circuit in case the district selector 175 is released before the called subscriber restores his receiver to the switehhoolt. In order that there may be no talking circuit from the jack 107 during connection with the trunk circuit through the brushes of district selector 17.5, relay 110 at its outer armature and contact opens the connection of conductor 118 to the tip contact of jack 107.

Following the seizure of the trunk circuit 117, 118 by the district selector 175, the sender associated with the district selector functions in the manner well known in the art to transmit impulses to the call indicator at the terminating oflice, for recording the designation of the desired line. From this point, the circuits function in substantially the same manner as hereinbefore described.

lVh-at is claimed is:

1. In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connect-ion to said line, means for connecting with the outgoing end of said trunk circuit at a plurality of points, means for rendering said trunk circuit busy at all of said points when connect-ion is made therewith, and means for maintaining said busy condition until either the called subscriber restores his receiver or said connection extending means is disconnected from said line.

2. In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means for connecting with the outgoing end of said trunk circuit at a plurality of points, a plurality of mutually locking relays for rendering said trunk circuit busy at all of said points when a connection is made therewith, and means under the control of the called subscriber for unlocking said relays to render said trunk circuit available for other connections.

3. In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means for connecting with the outgoing end of said trunk circuit at a plurality of points, a plurality of mutually locking relays for rendering said trunk circuit busy at all of said'points when connection is made therewith, and a relay operable over said trunk circuit upon the restoration of the subscribers receiver to its switchhook for unlocking said relays to render said trunk circuit available for other connections.

4. In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means for connecting with the outgoing end of said trunk circuit at a plurality of points, a plurality of mutually locking relays for rendering said trunk circuit busy at all of said points when connection is made therewith, and a relay operable over said trunk circuit upon the restoration of the subscribers receiver to its switchhook for shunting down one of said relays whereby said relays become unlocked to render said trunk circuit again available for other connections.

5. in a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means for connecting with the out-going end of said trunk circuit at a plurality of points, a plurality of mutually locking relays for rendering said trunk circuit busy at all of said points when connection is made therewith, a relay for unlocking said relays to render said trunk circuit again available for other connections, means for bridging said relay across said trunk circuit upon disconnection from said trunk circuit at its outgoing end, and means operative following the restoration of the subscribers receiver to its switehhcok for operating said bridged relay.

6. In a telephone exchange system, a called line, a trunk circuit terminating in means for extending a connection to said line, means at the terminating end of said trunk circuit for applying ringing current to said line in accordance with a predetermined code, an operators cord circuit for connection with said line, means operative following the connection of said cord circuit with said line for cutting off said ringing current, a second operators cord circuit for connecting with the outgoing end of said trunk circuit, and means at said second cord circuit for initiating the operation of said ringing current applying means and for applying ringing current at will to said line following the initial response of said first operator. i f

7 In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means at the terminating end of said trunk circuit for applying ringing current to said line, a disconnect signal, means for operating said signal upon the opening of the circuit over said trunk, an operators cord circuit for connecting with the outgoing end of said trunk circuit, a key at said cord circuit for initiating the operation of said ringing current applying means, and a high impedance bridge for said trunk circuit at the outgoing end thereof for preventing the operation of said signal due to the temporary opening of the circuit over said trunk in the operation of said key.

8. In a telephone exchange system, a subscribers line, a trunk circuit terminating in means for extending a connection to said line, means at the terminating end of said trunk circuit for applying ringing current to said line, a disconnect signal, means for operating said signal upon the opening of the circuit over said trunk, an operators cord circuit for connecting with the outgoing end of said trunk circuit, a ringing key at said cord circuit for opening said cord circuit and for applying ringing current over said trunk circuit, means responsive to said ringing current for initiating the operation of said ringing current applying means, and a high impedance brid 'e for said trunk circuit at the outgoing end thereof to hold said signaling operating means operated for preventing the operation of said signal due to the temporary opening of the circuit over said trunk in the operation of said key.

9. In a telephone exchange system, subscribers lines, a trunk circuit terminating in manually controlled means for extending connections to said lines, an indicator associated with said means, an operators cord circuit for connecting with the outgoing end of said trunk circuit, a sender associated with said operators position for operating said indicator to, record the designation of a desired subscribers line, selector switches for connecting with the outgoing end of said trunk circuit, a sender for controlling said switches and for operating said indicator, means for rendering said trunk circuit busy when connection is made therewith by either said operators cord circuit or said switches, and means for removing said busy condition upon disconnection from said trunk circuit in response to the restoration of the called subscribers receiver or the disconnection of said extending means from the called line.

In witness whereof, I hereunto subscribe my name this 14th day of December A. D., 1923.

JACK F. DAHT 

